1. Technical Field
Embodiments of the present invention relate to a semiconductor device and specifically to a power semiconductor module where a power semiconductor chip is mounted.
2. Discussion of the Background
Semiconductor devices called power semiconductor modules, where plural power semiconductor chips such as insulating gate bipolar transistors (IGBTs), power metal-oxide-semiconductor field-effect transistors (MOSFETs), and freewheeling diodes (FWDs) are mounted, and which are used for electrical power control in inverters or the like, have been widely used in industrial fields.
In the power semiconductor module, in order to protect an insulating substrate having a wiring thin film and the power semiconductor chips bonded on the wiring thin film by solder, which are arranged within a resin case, the resin case is sealed by filling a sealing resin such as silicone gel. With this silicone gel, an insulating property (dielectric breakdown voltage) within the resin case is ensured.
On the other hand, because the power semiconductor chips generate high heat during operation, it becomes important to enhance a cooling performance for the power semiconductor chips in ensuring a long-term reliability of the power semiconductor modules. To this end, in an existing power semiconductor module, a base plate made of metal is bonded by solder on a reverse side of the insulating substrate for holding the power semiconductor chips, and further cooling fins are added, thereby to enhance the cooling performance.
FIG. 4 is a principal part cross-sectional view of an existing power semiconductor module 100. Power semiconductor chips 1 are bonded by solder (not illustrated) on a wiring thin film 2 provided on an upper surface of an insulating substrate 3 made of an insulating material such as alumina. Incidentally, the wiring thin film 2 is generally made of copper or copper alloy.
In addition, because electrical power is controlled by flowing electric current in a thickness direction of the semiconductor chip in the power semiconductor chips 1, there exist electrodes having different functions on respective chip surfaces. Thus, it is required to provide a different wiring on a surface of the semiconductor chip, the surface being opposite to a surface that faces the wiring thin film. As this different wiring, bonding wires 6, which are generally made of copper, aluminum, gold, or the like, are used.
Moreover, in order to ensure the cooling performance for the heat generated from the power semiconductor chips 1, the insulating substrate 3 and a metal base plate 5, which is made of copper, aluminum, or the like, are firmly attached to each other. Incidentally, when the solder is used as a firmly attaching measure, a metal thin film 4 made of copper, copper alloy, or the like becomes further necessary on the reverse side of the insulating substrate 3, as illustrated in FIG. 4.
After the above configuration and various wirings (not illustrated) are incorporated into a case frame 7, a sealing resin A made of silicone gel is filled thereinto, and then a case lid 8 is attached, thereby to complete the power semiconductor module 100.
However, in the existing power semiconductor module 100, while the insulating substrate 3 and the metal base plate 5 are capable of contributing to heat dissipation because of their high thermal conductivities as illustrated in FIG. 5, the silicone gel for sealing (sealing resin A) scarcely contributes to the heat dissipation because of its low thermal conductivity. Therefore, when a large amount of heat is generated from the power semiconductor chips 1, such as in a case where the power semiconductor chips 1 are used at a high current, a sufficient cooling performance is not ensured, which may lead to a problem in a long-term reliability.
So, it has been known that by adding a filler having an insulating property and a high thermal conductivity to the silicone gel for sealing the cooling performance is improved, as disclosed in Japanese Publication JP-A-5-129474 and Japanese Publication JP-A-62-21249. A power semiconductor module 110 that has silicone gel, to which the filler is added, uses a sealing resin B whose thermal conductivity is improved by adding the filler such as silica, alumina, and diamond to the silicone gel of the filler, while maintaining the insulating property, as illustrated in FIG. 6. With this, in the power semiconductor module 110, because the heat dissipation from both surfaces of the power semiconductor chips 1 can be ensured, as illustrated in FIG. 7, the cooling performance is improved.
On the other hand, because an influence of thermal stress cannot be ignored in the silicone gel containing the thermally-conductive filler, compared with the silicone gel in the past, there may be caused a problem of peeling off in protrusion electrodes fixed and supported by the substrate or the semiconductor chip. So, as is a power semiconductor module 120 illustrated in FIG. 8, it has also been reported that the sealing resin A made of the silicone gel in the past is used in the vicinity of protrusion electrodes 11, and a sealing resin B containing the fillers there above is used (Japanese Publication JP-A-62-21249).